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Get the reporting, research and analysis behind on-air stories straight from the CNN Medical Unit, led by chief medical correspondent Dr. Sanjay Gupta.

Autism rates up despite removal of mercury from vaccines

• Most vaccine makers started removing mercury-based preservative thimerosal in '99 • Autism rates in California went up from 3 to 4 per 1,000 from 2004 to 2007 • Study authors say this proves no autism and mercury-based thimerosal link

Removal of thimerosal from most vaccines hasn't reduced the number of autism cases diagnosed in the state of California.

One doctor says the study "very clearly shows that autism did not arrive through a vaccine."

Overview

A new study published in the January 2008 issue of Archives of General Psychiatry found the prevalence of autism cases in California children continued to rise after most vaccine manufacturers started to remove the mercury-based preservative thimerosal in 1999, suggesting that the chemical was not a primary cause of the disorder. Researchers from the State Public Health Department found that the autism rates in children rose continuously during the study period from 1995 to 2007. The preservative, thimerosal, has not been used in childhood vaccines since 2001, except for some flu shots. The latest findings failed to convince some parents and advocacy groups, who have long blamed mercury, a neurotoxin, for the disorder.

Questions and answers

For years, parents have been concerned that a mercury-containing vaccine preservative may play a role in autism. But a study conducted in California found that autism rates increased even after thimerosal was removed from most vaccines. The study authors say this is evidence that thimerosal does not cause autism, although advocacy groups say it's too soon to determine whether autism rates have been affected. Do these findings suggest that autism isn't linked to mercury in vaccines?

Dr. Sanjay Gupta, chief medical correspondent: Let me explain. In 1999, manufacturers began removing thimerosal - which is a mercury-based preservative - from vaccines. Some people believed autism would decrease as a result, because they thought the two were connected. A new study in the Archives of General Psychiatry says this just didn't happen. Researchers looked at cases of autism in California after 1999. They reasoned that if mercury exposure in vaccines was a major cause of autism, the number of affected kids should have dropped after thimerosal was removed.

Just the opposite happened. From 2004 to 2007, when exposure to thimerosal dropped significantly for 3- to 5-year-olds, the autism rate continued to go up, from 3 per 1,000 children to 4 per 1,000 children in California.

A child psychiatrist who supported the study said it "very clearly shows that autism did not arrive through a vaccine." But advocacy groups say it's too soon to determine whether autism rates were affected by removing thimerosal from vaccines. The National Vaccine Information Center says the study doesn't include children under the age of 3, which they say is the only group that was never exposed to mercury in vaccines. It says thimerosal wasn't completely off the shelves until 2002 or 2003. Their main point is that mercury is a neurotoxin, so why take a chance by putting it in vaccines?

What do scientists think causes autism?

As many as one in every 166 children in this country is found to have autism, and doctors still don't know why. Doctors point to genetics and environment as culprits, but it could be more complicated than that. The latest research shows these children are not necessarily born with autism but with the potential to develop it. What exactly are these outside factors? It's hard to pinpoint. What we eat, what we breathe, what we drink -- all these things could play a role. Some doctors say the increase is due to a change in the way the condition is diagnosed kids who were once labeled mentally retarded are now being labeled as autistic.

What are possible signs of autism in your child?

Doctors are now looking for signs of autism in children as young as 18 to 24 months.

Some red flags that indicate your child may have autism: no babbling or pointing by 12 months, no single words by 16 months, no brief phrases by 24 months, loss of language or social skills. If you see any of these signs, the American Academy of Pediatrics recommends seeing a pediatric neurologist, developmental pediatrician or child psychologist.

What led companies to remove thimerosal from vaccines to begin with?

Several things pushed companies in this direction. Over the past decade, more and more attention was given to the health effects of mercury on humans. And then in the '90s, the CDC added new vaccines to the list of routine shots that children should get. Some of them used thimerosal as a preservative. This was happening while the government was trying to decrease our exposure to mercury. So the FDA began looking into the issue. In 1997, Congress passed a bill that mandated review of products containing mercury, which led manufacturers to begin removing thimerosal from vaccines two years later.

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CNN spoke with Dr. Eric Fombonne, a professor of psychiatry at McGill University in Montreal, Quebec, and an autism expert, who wrote a commentary accompanying the Archives of General Psychiatry study. Here are a few of his comments:

Fombonne: Several different studies, with different designs, performed in different countries have all failed to show a link between thimerosal and autism. This is particularly important in the U.S. context because so many more people believe thimerosal is implicated in autism [in the U.S.], than in other countries. Years ago, people made predictions -- by removing thimerosal, the number of cases of autism should decrease -- therefore showing that thimerosal is a cause of autism. This new study puts that idea in jeopardy. Similar studies have been done in Canada and Denmark with the same results: thimerosal was removed, but autism is still on the rise. This is a strong message; it very clearly shows, and reassures, that autism did not arrive through a vaccine. ...

All theories have been tested, there's lots of empirical data. We have an accumulation of studies -- science posits in accumulation. Now, focus can be placed on possible environmental or genetic factors, because thimerosal has been ruled out. Also, parents should not fear vaccines.

CNN also spoke with Dr. Pauline Filipek, at the University of California, Irvine, School of Medicine and the founder of OC Kids, a center for autism. Here are excerpts from the conversation.

CNN: What are your thoughts on this new study?

Filipek: I think it's fabulous. I think it's long overdue. I've been using the graphs that [show these rates] in my lectures. I have been showing when thimerosal was totally removed. Now there is a formal study to show there is no association ... with a lot more scientific validity. If there was a connection with the mercury, I should be almost out of business as an autism specialist by now, eight years later, because there should not be any more children with autism if it was caused by the mercury in the vaccine.

Some parents say this study presents a greatly oversimplified explanation of a very complex problem. ... The party line has been that mercury causes all of autism. This adds to the data that disprove that.

CNN: Is it safe to say this study proves there is no link between autism and vaccines?

Filipek: I don't think any one study would definitely prove that. But I think it gives formal credence to what the institute of medicine has said and what other studies have said. I think the data is now that much stronger.

CNN: How do you explain rising numbers?

Filipek: I can't explain it at this point in time. I think that it is a complex question with complex answers. Some of it is due to the broader diagnostic criteria, some of it is due to the greater recognition, but most certainly not all of it can be explained by those two.

CNN: The group also said that some vaccines containing thimerosal had expiration dates as late as 2005 and may have been used up to that point.

Filipek: ... We know there's a strong genetic component to autism, but that can't explain the increase. So there must be something else at work. ... We do know that environment can influence genetics. We are understanding more and more how the environment interplays or interacts with genetics. But that field is still very, very young, relatively speaking. If it was so simple as being mercury in the vaccines, it should be going away. Even if there were mercury in some vaccines, even if that statement's true, the rates should have plateaued. ... I think it's one more study that disproves the link.

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